Power transmission mechanism

ABSTRACT

An interruptible power transmission mechanism couples a drive source to a compressor. The power transmission mechanism has a pulley, which rotates in synchronism with the drive source, and a receiving member, which rotates in synchronism with the compressor. A limit spring couples the pulley and the receiving member such that they rotate together. When the load torque of the compressor exceeds a predetermined value, the diameter of the limit spring is decreased so that the limit spring engages a rib provided on the receiving member. Then, the deformation of the limit spring in the radial direction is locally restricted, causing stress at a specific portion of the limit spring to increase rapidly. As a result, the limit spring is reliably broken at a torque near the desired load torque, thus interrupting power in a desirable manner.

BACKGROUND OF THE INVENTION

The present invention relates to a power transmission mechanism providedbetween a drive source and a driven machine. More specifically, thisinvention relates to a power transmission mechanism that interruptstransmission between a drive source and a driven machine when an excessload torque is produced by the driven machine.

In general, a power transmission mechanism is provided between a drivesource, such as an engine or a motor, and a driven machine, such as acompressor. When an abnormality (e.g., seizure) occurs in the drivenmachine, the power transmission mechanism positively shuts off powertransmission between the drive source and the driven machine to preventthe excess load torque from affecting the drive source.

For example, Japanese Unexamined Patent Publication (KOKAI) Hei No.8-319945 discloses a clutchless compressor in which a pulley, which isfitted over the end portion of the rotary shaft, is driven by an engine.The pulley, or power transmission mechanism, has a plurality of arcuateholes arranged at predetermined intervals on an imaginary circle aboutthe axis of the rotary shaft. The portions between adjacent pairs ofholes form break portions. When the rotary shaft is unable to rotate dueto an abnormality in the internal mechanism of the compressor and a loadtorque equal to or greater than a predetermined value acts on the breakportion, the break portion breaks. Consequently, the power transmissionto the rotary shaft from the engine is cut off.

According to the power transmission mechanism of the aforementionedpublication, the break portion does not always fully break when the loadtorque reaches the predetermined value. Specifically, for example, thefailure stresses of the individual members, if they are of the same kindor are the same member, are not quite the same and have a certainvariation. It is therefore actually very hard to reliably break thebreak portion in the vicinity of a load torque where breaking isexpected in individual power transmission mechanisms that have suchindividual differences. Accordingly, a simple structure that has a breakportion merely provided at a part of the pulley is not practical, andthere is no guarantee that breakage will occur as expected.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a powertransmission mechanism that reliably breaks in the vicinity of a desiredload torque to accomplish suitable power cutoff.

To achieve the above object, this invention provides a powertransmission mechanism for coupling a drive source to a driven machinein an interruptible manner. The power transmission mechanism includes afirst rotary body, which rotates in synchronism with the drive source,and a second rotary body, which rotates in synchronism with the drivenmachine. Coupling means couples the first rotary body and the secondrotary body in a synchronously rotatable manner. Engagement meansengages with the coupling means when the load torque of the drivenmachine exceeds a predetermined value. The engagement means, which is inengagement with the coupling means, increases stress at a specificportion of the coupling means to break the coupling means.

A power transmission mechanism provided according to another aspect ofthis invention includes a first rotary body, which rotates insynchronism with the drive source, and a second rotary body, whichrotates in synchronism with the driven machine. Coupling means couplesthe first rotary body and the second rotary body in a synchronouslyrotatable manner. As the load torque of the driven machine increases,the stress of the coupling means increases. Engagement means engageswith the coupling means to increase the ratio of the change in thestress of the coupling means to the change in the load torque of thedriven machine. The engagement means engages with the coupling means tobreak the coupling means when the load torque of the driven machineexceeds a predetermined value.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a variable displacement compressoraccording to one embodiment of the present invention;

FIG. 2(A) is a front view of a power transmission mechanism equipped inthe compressor in FIG. 1;

FIG. 2(B) is a cross-sectional view taken along the line 2B—2B in FIG.2(A);

FIG. 3 is a cross-sectional view of a receiving member taken along theline 3—3 in FIG. 2(A);

FIG. 4(A) is a front view of a boss of a pulley;

FIG. 4(B) is a perspective view of the boss of the pulley;

FIG. 5 is an explanatory diagram showing the state of a coil spring in apower transmitting state;

FIG. 6 is an explanatory diagram showing the state of the coil springimmediately before breaking; and

FIG. 7 is a graph illustrating the relationship between the load torqueof a compressor and stress which acts on a limit spring.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of the present invention, as embodied in a variabledisplacement compressor of an air-conditioning system for a vehicle,will now be described with reference to FIGS. 1 through 7. Thecompressor in this embodiment is called a clutchless compressor becauseit does not require a clutch mechanism, such as an electromagneticclutch, between itself and an engine, or drive source. A powertransmission mechanism according to this invention is used in place ofsuch a clutch mechanism and has two functions, which are powertransmission in a normal mode and power cutoff in an emergency mode.

As shown in FIG. 1, the vehicular air-conditioning system comprises arocking swash plate type variable displacement compressor 10, anexternal refrigeration circuit 30 and a controller 34, which performsgeneral control of the air-conditioning system. The externalrefrigeration circuit 30 has, for example, a condenser 31, a temperaturetype expansion valve 32 and an evaporator 33. The external refrigerationcircuit and the compressor 10 constitute a refrigeration cycle.

The compressor 10, or driven machine, has a cylinder block 11, a fronthousing 12, which is connected to the front end face of the cylinderblock 11, a valve plate 14 and a rear housing 13, which is connected tothe rear end face of the cylinder block 11 through the valve plate 14.The cylinder block 11, the front housing 12, the rear housing 13 and thevalve plate 14 constitute the housing of the compressor 10.

A crank chamber 15 is defined between the front housing 12 and thecylinder block 11. A drive shaft 16 is rotatably supported by the fronthousing 12 and the cylinder block 11. In the crank chamber 15, a lugplate 18 is fixed to the drive shaft 16. The lug plate 16 contacts theinner wall of the front housing 12 via a thrust bearing 17. A swashplate 19 as a drive plate is supported in the crank chamber 15 by thedrive shaft 16 such that the swash plate 19 can tilt and slide in theaxial direction. The swash plate 19 is coupled to the lug plate 18 via ahinge mechanism 20. The lug plate 18 and the hinge mechanism 20 allowthe swash plate 19 to slide and tilt with respect to the drive shaft 16and rotate integrally with the drive shaft 16.

A plurality of cylinder bores 11 a (only one shown in FIG. 1) arelocated in the cylinder block 11. The cylinder bores 11 a are providedat equal intervals on a circle centered on axial line L of the driveshaft 16. A one-headed piston 21 is retained in each cylinder bore 11 ain a reciprocatable manner. One end of each piston 21 is coupled to theperipheral portion of the swash plate via a pair of shoes 22. In eachcylinder bore 11 a, a compression chamber is defined between the endface of the piston 21 and the valve plate 14. As the drive shaft 16rotates, the swash plate 19 rotates and each piston 21 reciprocates inthe cylinder bore 11 a.

A suction chamber 25 and a discharge chamber 26 are defined in the rearhousing 13. The suction chamber 25 and the discharge chamber 26 areconnected together by the external refrigeration circuit 30. The valveplate 14 is constructed by stacking at least three metal plates. Thevalve plate 14 has suction ports and discharge ports in association withthe individual cylinder bores 11 a. The valve plate 14 further has inletvalves 14 a, which are flapper valves, corresponding to the individualsuction ports and discharge valves 14 b, which are flapper valves,corresponding to the individual discharge ports. When the piston 21moves from the top dead center to the bottom dead center, therefrigerant gas in the suction chamber 25 pushes the inlet valve 14 aopen and flows into the cylinder bore 11 a. When the piston 21 movesfrom the bottom dead center to the top dead center, the refrigerant gasin the cylinder bore 11 a is compressed to a predetermined pressure andpushes the discharge valve 14 b open from the discharge port and isdischarged into the discharge chamber 26.

A supply passage 23, which connects the crank chamber 15 to thedischarge chamber 26, is provided in the cylinder block 11, the valveplate 14 and the rear housing 13. Located in the supply passage 23 is adisplacement control valve 24, which is incorporated into the rearhousing 13. The displacement control valve 24 is, for example, anelectromagnetic valve having a solenoid 24 a, a valve body 24 b and aport 24 c. The port 24 c constitutes a part of the supply passage 23.The controller 34 supplies a current to the solenoid 24 a. When thesolenoid 24 a is excited, the valve body 24 b closes the port 24 c, andwhen the solenoid 24 a is deexcited, the valve body 24 b opens the port24 c.

A support hole 11 b which supports the rear end of the drive shaft 16 isformed in nearly the center of the cylinder block 11. A pressure-releasepassage 16 a is formed in the drive shaft 16 to extend along the axis L.The pressure-release passage 16 a has an inlet, which opens into thecrank chamber 15, and an outlet, which opens into the support hole 11 b.The support hole 11 b is connected to the suction chamber 25 via arestriction hole 27, which passes through the cylinder block 11 and thevalve plate 14. The pressure-release passage 16 a, the support hole 11 band the restriction hole 27 serve as a bleeding passage for allowing therefrigerant gas in the crank chamber 15 to escape into the suctionchamber 25.

The discharge displacement of the compressor 10 is changed by adjustingthe pressure in the crank chamber 15 (crank pressure) with thedisplacement control valve 24. Specifically, as the controller 34controls the current supply to the control valve 24, the position of thecontrol valve 24 is adjusted. As a result, the relationship between theamount of the gas that is supplied into the crank chamber 15 from thedischarge chamber 25 via the supply passage 23 and the amount of gasthat flows into the suction chamber 26 from the crank chamber 15 via thebleeding passage changes, thus adjusting the crank pressure.

When the crank pressure rises, the inclination angle of the swash plate19 becomes smaller and the stroke of each piston 21 becomes smaller,thus reducing the discharge displacement. When the crank pressurebecomes lower, on the other hand, the inclination angle of the swashplate 19 becomes larger and the stroke of each piston 21 becomes larger,thus increasing the discharge displacement.

The controller 34 determines the level of the cooling load in a vehiclebased on detection information from various sensors (not shown),including a temperature sensor provided on the evaporator 33, andcontrols the current supply to the control valve 24 in accordance withthe cooling load. Consequently, the angle of the control valve 24changes and the crank pressure or the inclination angle of the swashplate 19 is determined in accordance with the inclination angle, so thatthe discharge displacement of the compressor 10 is adjusted to match thecooling load. As apparent from the above, the discharge displacement(compression performance) undergoes feedback control based on thecontrol of the inclination angle of the swash plate 19 according to achange in cooling load.

As shown in FIG. 1, the maximum inclination angle of the swash plate 19is restricted when a stopper 19 a provided on the swash plate 19 abutsagainst the lug plate 18. In addition, the minimum inclination angle ofthe swash plate 19 is restricted as the swash plate 19 abuts on arestriction ring 28 provided on the drive shaft 16. In general, theminimum inclination angle is set slightly larger than 0° so that thestroke of the piston 21 does not become zero.

The power transmission mechanism provided in the compressor 10 will nowbe described. As shown in FIGS. 1, 2(A) and 2(B), a support cylinder 41extends from the front end of the front housing 12. An angular bearing42 is provided around the support cylinder 41. A pulley 43, or a firstrotary body, is fixed to the outer race of the angular bearing 42.Therefore, the pulley 43 is supported to rotate with respect to thesupport cylinder 41. The pulley 43 is coupled to a vehicular engine 35,or a drive source, via a power transmission belt 44, such as a V belt.The pulley 43 has a boss 43 a, which is attached to the outer race ofthe angular bearing 42, an outer ring 43 b, on which the belt 44 iswrapped, a disc portion 43 c, which connects the boss 43 a to the outerring 43 b. An annular recess (or an annular groove) 46 is located in thearea bounded by the boss 43 a, the outer ring 43 b and the disc portion43 c.

A receiving member 50 is fixed to the front end of the drive shaft 16 bya bolt 47. Therefore, the drive shaft 16 and the receiving member 50rotate together. The drive shaft 16 and the receiving member 50 form asecond rotary body.

FIG. 3 shows the cross section of the receiving member 50 along the line3—3 in FIG. 2(A). As shown in FIGS. 2(A), 2(B) and 3, the receivingmember 50 has a cylinder portion 51, which is fitted over the outersurface of the front end of the drive shaft 16, and a pair of plate armportions 52, which extend from the outer end portion of the cylinderportion 51 in the radial direction. The plate arm portions 52 arearranged linearly, on opposite sides of the bolt 47. That is, the pairof plate arm portions 52 are angularly spaced apart by 180° about theaxis of the receiving member 50. A step portion 52 a is formed at thedistal end of each plate arm portion 52.

The receiving member 50 further has a pair of ribs 53 that extend in theradial direction. The ribs 53 constitute engagement means. The pair ofribs 53 is provided in association with the pair of plate arm portions52. The ribs 53 are each provided on the bottom surface of theassociated plate arm portion 52.

As shown in FIG. 2(A), the distal end (the outermost end in the radialdirection) of each rib 53 extends to the position of the outer surfaceof the boss 43 a of the pulley 43. In other words, the distance from theaxial center of the receiving member 50 to the distal end of the rib 53coincides with the radius of the outermost periphery of the boss 43 a.

As shown in FIGS. 2(A) and 2(B), a limit spring 60 as coupling means isplaced around the boss 43 a of the pulley 43. The limit spring 60comprises first and second torsion coil springs 601 and 602. Both coilsprings 601 and 602 are made of metal. Each of the coil springs 601, 602has a body portion 61 formed in a helical shape and a first end portion62 and a second end portion 63, which are located at the ends of thebody portion 61. In FIG. 5, only one of the coil springs 601 and 602 isshown.

As shown in FIGS. 2(A) and 5, the first and second end portions 62 and63 of each torsion coil spring 601, 602 are positioned outside thehelical cylinder that the body portion 61 defines. As shown in FIGS.2(A) and 2(B), each first end portion 62 is fixed by rivets to the discportion 43 c at a corner portion which is formed by the inner surface ofthe outer ring 43 b of the pulley 43 and the disc portion 43 c. Eachsecond end portion 63 is fixed to the step portion 52 a of the plate armportion 52 of the receiving member 50 by rivets.

The body portion 61 of each of the torsion coil springs 601, 602 is heldbetween the outer surface of the boss 43 a and the inner surface of theouter ring 43 b without contacting them. That is, with the first andsecond end portions 62 and 63 respectively fixed to the disc portion 43c and the plate arm portion 52, the radius of the helical cylinderdefined by the body portion 61 is set in such a way as to be greaterthan the radius of the outer surface of the boss 43 a and smaller thanthe inside diameter of the outer ring 43 b. The outside diameter of thecylindrical boss 43 a is smaller than at least the diameter of each ofthe coil springs 601, 602 in the normal state.

Each body portion 61 is wound around the boss 43 a approximately two andhalf helical turns. Note that the portion of the body portion 61 thatfaces the outer surface of the boss 43 a ranges from the first endportion 62 to about one and half turns to about two turns, and theremaining portion close to the second end portion 63 (about one turn toabout a half turn) is located forward of the distal end of the boss 43a, as shown in FIG. 2(B). That is, the limit spring 60 has a firstportion (rear half) arranged around the boss 43 a to face the boss 43 ain the radial direction and a second portion (front half) which does notface the boss 43 a in the radial direction. The ribs 53 of the receivingmember 50 are also located forward of the distal end of the boss 43 a.In FIGS. 2(A), 5 and 6, the annular end face 48 of the distal end of theboss 43 a has a flecked pattern to help understand the drawings.

As shown in FIGS. 4(A), 4(B) and 5, the annular end face 48 is providedwith engagement projections 491 and 492 (only one of the engagementprojections 491 and 492 is shown in FIG. 5). The engagement projections491 and 492 are formed to extend from the annular end face 48. Theengagement projections 491 and 492 are located at positions of 180° fromeach other about the axis of the boss 43 a. The first engagementprojection 491 is associated with the coil spring 601 and the secondengagement projection 492 is associated with the coil spring 602.

For example, the first coil spring 601 has a layout relation with thefirst engagement projection 491 and one of the ribs 53 as shown in FIG.5 with the end portions 62 and 63 fixed to the pulley 43 and thereceiving member 50. FIG. 6 shows the state in which the first coilspring 601 is on the verge of breaking as a result of relative rotationbetween the pulley 43 and the receiving member 50 caused by excess loadtorque generated in the inner mechanism of the compressor. At this time,the first engagement projection 491 and the rib 53 are arranged oppositeto each other (angularly separated by almost 180°). The second coilspring 602, the second engagement projection 492 and the rib 53, whichworks in cooperation with the projection 492, have a layout relationshipsimilar to that described above.

Each of the engagement projections 491 and 492 serves as a hook portionto prevent a part of the spring wound around the outer surface of theboss 43 a from coming off that outer surface when the diameter of theassociated coil spring 601, 602 is reduced.

As shown in FIG. 2(A), the end portions 62 and 63 of the torsion coilspring 601 and those of the torsion coil spring 602 are located atangularly separated positions different from each other by approximately180° about the bolt 47. The torsion coil springs 601 and 602 are joinedto constitute the single limit spring 60. Therefore, the limit spring 60serves as a double torsion coil spring having two wires wound to beparallel to each other.

As shown in FIG. 2(B), the rear half of the limit spring 60 is retainedin the annular recess 46 of the pulley 43, and the front half of thelimit spring 60 is exposed outside of the annular recess 46. The limitspring 60 is located, compressed in the axial direction, between thedisc portion 43 c of the pulley and the receiving member 50. Therefore,the restoring force of the limit spring 60 urges the receiving member 50and the drive shaft 16 forward.

As apparent from the above, the pulley 43 is coupled to the receivingmember 50 and the drive shaft 16 in a power transmittable manner via thelimit spring 60, which includes two torsion coil springs 601 and 602.The limit spring 60 therefore serves as a coupling means that couplesthe first rotary body and the second rotary body in a synchronouslyrotatable manner.

The operation of this embodiment will now be discussed with reference toFIGS. 5 to 7. Note that FIGS. 5 and 6 omit the receiving member 50 andshow only one of the two coil springs 601 and 602 for easierunderstanding.

The power of the engine 35 is normally transmitted to the drive shaft 16via the belt 44, the pulley 43, the limit spring 60 (torsion coilsprings 601 and 602) and the receiving member 50. That is, the supplytorque of the engine 35 is balanced with the load torque of thecompressor 10, and the pulley 43 and the drive shaft 16 synchronouslyrotate with the angular velocity ω1 of the pulley 43, which is equal tothe angular velocity ω2 of the receiving member 50, and the drive shaft16 as shown in FIG. 5. In this case, the body portion 61 of each torsioncoil spring 601, 602 is kept separated from the outer surface of theboss 43 a of the pulley.

In accordance with the power transmission to the drive shaft 16, theswash plate 19 coupled to the drive shaft 16 causes the individualpistons 21 to reciprocate. The pistons 21 perform suction andcompression of the refrigerant gas. In accordance with this work (loadcondition), a load torque in the opposite direction to the rotationaldirection of the pulley 43 acts on the drive shaft 16 and the receivingmember 50. If the load torque does not exceed a predetermined limitvalue and is not large enough to impart an undesirable influence on theengine 35, however, the power transmission to the receiving member 50and the drive shaft 16 from the pulley 43 via both coil springs 601 and602 is maintained. As long as this power transmission is maintained,even if the load torque varies under the predetermined limit value dueto a phase shift of the pressure change in each cylinder bore 11 a, avariation in the compression load or the like, such a variation in loadtorque is sufficiently accommodated by the spring elasticity of the coilsprings 601 and 602.

When some kind of problem (e.g., seizure) occurs inside the compressorand the load torque of the compressor 10 exceeds the predetermined limitvalue, on the other hand, a difference between the angular velocity ω1of the pulley 43 and the angular velocity ω2 of the receiving member 50and the drive shaft 16 (see FIG. 6; ω2′<ω1) occurs. That is, the pulley43 and the receiving member 50 and the drive shaft 16 do not rotatesynchronously. Specifically, while the first end portions 62 of the coilsprings 601 and 602 coupled to the pulley 43 try to stay in synchronousrotation with the pulley 43, the second end portions 63 coupled to thereceiving member 50 strongly resist synchronous rotation with the pulley43, producing an angular velocity difference (ω1−ω2′) between the ends62 and 63.

This angular velocity difference deforms each coil spring 601, 602 suchthat its diameter decreases. As a result, as shown in FIG. 6, the rearhalf of the body portion 61 of the coil spring 601 (or 602) is woundaround the outer surface of the boss 43 a of the pulley 43 tightly and apart of the front half of the body portion 61 abuts against the distalend of the rib 53. When the rear half of the body portion 61 is woundaround the outer surface of the boss 43 a, further deformation isrestricted.

Based on the angular velocity difference between the pulley 43 and thereceiving member 50, the engagement projection 491 (or 492) ispositioned as shown in FIG. 6 with respect to the rib 53. As twisting isfurther applied to each of the coil springs 601, 602 in the direction ofreducing its diameter, the boundary portion between the front half andthe rear half of the body portion 61 is bent inward of the cylinderdefined by the outer surface of the boss 43 a at the engagementprojection 491 (or 492) and the portion of the body portion 61 that isin contact with the rib 53 is further bent sharply. As a result, stressdue to the twisting of each of the coil springs 601, 602 concentratesparticularly at the portion in contact with the rib 53, so that the bodyportion 61 finely breaks at that location.

This embodiment uses two coil springs 601 and 602, and if one coilspring breaks, all the load torque is applied to the remaining coilspring so that the remaining coil spring breaks immediately. When anexcess load torque that exceeds a predetermined limit value is produced,both coil springs 601 and 602 break almost simultaneously, so that powertransmission to the drive shaft 16 from the engine 35 is positivelydiscontinued.

FIG. 7 is a graph illustrating the relationship between the torqueapplied to the limit spring 60 (coil springs 601, 602) from thecompressor (i.e., load torque) and the stress that acts on the limitspring 60. In this graph, the solid line indicates the characteristicsof the power transmission mechanism according to this embodiment, andthe two-dot chain line indicates the characteristics of a comparativeexample equivalent to the structure of the power transmission mechanismof this embodiment except that the pair of ribs 53 and the pair ofengagement projections 491 and 492 are not present. Because the coilsprings in use in both cases are the same, a range F from the upperlimit to the lower limit of the stress (rupture stress) that is neededto break the body portion 61 is the same in both cases.

Since the line representing the characteristics of the comparativeexample has the same slope over the entire range of the applied torque,a range T2 of the applied torque corresponding to the rupture stressrange F also becomes relatively wide as shown in FIG. 7. In contrast,the slope of the line representing the characteristics of thisembodiment suddenly increases at a transition point B. That is, thetransition point B indicates the time when the body portion 61 contactsthe distal end of the rib 53. In the range of the applied torque beforethe transition point B, the body portion 61 and the rib 53 are not incontact with each other, and the slope of the characteristic line doesnot differ between this embodiment and the comparative example.

In this embodiment, after the body portion 61 contacts the distal end ofthe rib 53, however, the stress caused by the load torque concentratesat the point of contact with the rib 53 so that the stress tends to risesharply from there. Since the aforementioned rupture stress range Fcorresponds to the torque range after the transition point B where theslope of the characteristic line is large, the range T1 for the appliedtorque corresponding to that stress range F is relatively narrow(T1<T2). Therefore, that the range of the load torque for breaking thespring is narrower in this embodiment than in the comparative exampleand power transmission can positively be cut off when the load torque ofthe compressor approximately reaches the expected limit value (i.e., thebreak-expected torque).

This embodiment has the following effects.

The provision of the ribs 53 narrows the range T1 of the load torquecorresponding to the rupture stress range F of both coil springs 601 and602 so that the coil springs 601 and 602 can be broken with certainly atthe load torque at which breaking is expected, thus adequatelyaccomplishing power cutoff. It is therefore possible to guaranteeprotection of the engine 35 or the like against excess load torque.

Until the load torque of the compressor reaches the break-expectedtorque, the rear half of the twisted coil springs 601, 602 are be woundaround the outer surface of the boss 43 a. During this period, each ofthe coil springs 601, 602 and the boss 43 a rotate synchronously, sothat there is no abnormal sound produced by the winding, and no wear orthe like occurs between the coil springs 601, 602 and the boss 43 a.

When the coil springs 601, 602 are twisted further by the load torquefrom the state where the rear half of the coil springs 601, 602 arewound around the outer surface of the boss 43 a, a part of each coilspring 601, 602 is hooked on the associated engagement projection 491,492 of the boss 43 a and is further bent there. The presence of theengagement projection 491, 492 as a hook prevents the rear halves of thecoil springs 601, 602, which are wound around the outer surface of theboss 43 a from coming off the boss 43 a when twisting is furtherapplied. Therefore, the twisting action caused by the load torque on thefront halves of the coil springs 601, 602, which are located in front ofthe boss 43 a, is concentrated, so that even a slight increase in loadtorque increases the amount of deformation of the coil spring 601, 602at the point where the rib 53 makes contact. In this sense, theengagement projections 491 and 492 are means for aiding the breakingaction of the ribs 53.

The limit spring 60 includes a plurality of coil springs 601, 602. Theend portions 62 and 63 of the coil spring 601 and those of the coilspring 602 are coupled to other members at equal angular distances(i.e., angular positions different by 180°). During power transmissionfrom the engine 35, therefore, a moment that tilts the drive shaft 16with respect to the axis L is not produced and the rotation of thereceiving member 50 and the drive shaft 16 is stable, and torque istransmitted efficiently. Since the two torsion coil springs 601 and 602support each other, the postures of the coil springs 601 and 602 arestable when the two coil springs 601 and 602 are combined.

If a design that allows the first end portion 62 of each coil spring601, 602 to be engageable with and disengageable from the inner surfaceportion of the outer ring 43 b of the pulley 43 is employed, wear mayoccur at the engagement location. Because both end portions 62 and 63 ofeach coil spring are secured to the respective members in thisembodiment, by way of contrast, there is no need to consider wear.

Since the metal coil springs 601 and 602 are means that couples thefirst rotary body and the second rotary body, it is possible to set thespring constant of the limit spring 60 considerably low (morespecifically, lower than the spring constant of an ordinary rubberdamper). This makes it possible to set the resonance frequency of thepower transmission system lower than the minimum frequency of avariation in the load torque that occurs in the compressor 10, or thedriven machine. It is therefore possible to reduce noise and abnormalvibration due to resonance based on the load torque variation and toprevent the inner mechanism of the compressor 10 from being damaged(seeJapanese Patent Application No. Hei 9-30075 filed by the presentapplicant for more details).

Unless the load torque produced by the compressor exceeds apredetermined limit value, variation in the torque that acts on thedrive shaft 16 can be suppressed by the twisted deformation of the limitspring 60. That is, the limit spring 60 also serves as a damper.

Because the power transmission mechanism of this embodiment does notrequire a vibration isolating member such as a rubber cushion, it hasfewer components and is simple.

The intervening limit spring 60, which is axially compressed between thepulley 43 and the receiving member 50, also urges the drive shaft 16together with the receiving member 50 frontward. This suppressesrattling of the drive shaft 16 in the axial direction. It is thereforeunnecessary to consider the provision of a special spring member or thelike for urging the drive shaft 16 in the axial direction. The limitspring 60 therefore also contributes to simplifying of the compressor10.

The above-described embodiment may be modified in the following forms.

The portion of the body portion 61 of each coil spring 601, 602 thatcontacts the rib 53 and its neighboring portion may be quenched by meanssuch as a laser to make that portion harder than the other portions. Theincrease in hardness of the coil spring 601, 602 makes that portion moresusceptible to stress fracture.

Although the portion of each of the coil springs 601, 602 that is closerto the receiving member 50 (part of the front half) is designed to breakin the embodiment in FIGS. 1 through 6, the portion of the coil spring601, 602 that is closer to the pulley (part of the rear half) may bedesigned to break.

Even if the part of the front half of each coil spring 601, 602 isintended to break, it is unnecessary to make the ribs 53 as a maincutoff member and make the engagement projections 491 and 492 as abreak-aiding member. The shapes of the engagement projections 491 and492 may be changed so that the engagement projections 491 and 492themselves become a main cutoff member.

The engagement projections 491 and 492 may be omitted.

It is not essential to angularly separate the rib 53 and the engagementprojection 491 (or 492) opposite to each other by 180°, sandwiching theaxial center of the boss 43 a as shown in FIG. 6, when the compressor isoverloaded. On the verge of breaking (see FIG. 6), they may have such alayout relation that the angle θ that is formed by the rib 53, the axialcenter of the boss 43 a and the engagement projection 491 (or 492) isless than 180°. However, note that if the angle θ is too small, thecooperative and synergetic effect of the rib 53 and the engagementprojection 491, 492 may become low.

The shape of the engagement projection 491, 492 is not limited to theone shown in FIG. 4, but it may be a hook pin protruding from theannular end face 48 of the boss 43 a. Alternatively, the hook portionmay be protrude from the outer surface of the boss 43 a.

One of the two torsion coil springs 601 and 602 may be omitted so thatthe limit spring 60 is comprised of a single coil spring. Alternatively,the limit spring 60 may be constructed by using three or more coilsprings. That is, the limit spring 60 includes at least one coil spring.

The compressor 10 in FIG. 1 may be additionally provided with anelectromagnetic clutch.

What is claimed is:
 1. An interruptible power transmission mechanism forcoupling a drive source to a driven machine comprising: a first rotorthat is driven by the drive source; a second rotor that rotatessynchronously with the driven machine; a coupler that connects the firstrotor to the second rotor; and an abutment that contacts the coupler ata contact location when the load torque of the driven machine exceeds apredetermined value, wherein the abutment increases the stress in thecoupler at the contact location and promotes breakage of the couplerwhen the load torque of the driven machine exceeds the predeterminedvalue, wherein the ratio of the change in the stress in the coupler atthe contact location to the change of the load torque of the drivenmachine increases when the abutment contacts the coupler, and whereinthe coupler includes an elastic member that deforms in accordance withthe load torque of the driven machine.
 2. The power transmissionmechanism of claim 1, wherein the abutment resists deformation of thecoupler in an inward radial direction at the contact location.
 3. Thepower transmission mechanism of claim 1, wherein the coupler includes acoil spring.
 4. The power transmission mechanism of claim 3, wherein thecoil spring is axially compressed.
 5. The power of transmissionmechanism of claim 3, wherein the radius of the coil spring changes inaccordance with the load torque of the driven machine.
 6. The powertransmission mechanism of claim 3, wherein the first rotor includes apully and a boss, and the coil spring has a first section that surroundsthe boss and a second section that extends beyond the boss, wherein thecontact point is located on the second section.
 7. The powertransmission mechanism of claim 6, wherein the first section wrapsaround the boss and the abutment engages the second section when theload torque of the driven machine exceeds the predetermined value. 8.The power transmission mechanism of claim 7, wherein the boss includes ahook portion for engaging the coil spring when the load torque of thedriven machine exceeds the predetermined value.
 9. The powertransmission mechanism of claim 1, wherein the abutment is a rib formedon the second rotor.
 10. The power transmission mechanism of claim 9,wherein the driven machine has a drive shaft and a second rotor includesa member fixed to the drive shaft, and the rib is formed on the member.11. The power transmission mechanism of claim 1, wherein the couplerincludes a plurality of coaxial coil springs, wherein each coil springhas a first end fixed to the first rotor an a second end fixed to thesecond rotor, and the first ends are spaced apart at equal angularintervals, and the second ends are spaced apart at equal angularintervals.
 12. The power transmission mechanism of claim 1, wherein thecoupler includes a coil spring, and wherein the abutment is locatedinside the coil spring.
 13. The power transmission mechanism of claim12, wherein the abutment is fixed to the second rotor.
 14. Aninterruptible power transmission mechanism for coupling a drive sourceto a driven machine comprising: a first rotor that is driven by thedrive source; a second rotor that rotates synchronously with the drivenmachine; a coupler that connects the first rotor to the second rotor;and an abutment that contacts the coupler at a contact location when theload torque of the driven machine exceeds a predetermined value, whereinthe abutment increases the stress in the coupler at the contact locationand promotes breakage of the coupler when the load torque of the drivenmachine exceeds the predetermined value, wherein the ratio of the changein the stress in the coupler at the contact location to the change ofthe load torque of the driven machine increases when the abutmentcontacts the coupler, and wherein the abutment resists deformation ofthe coupler in a inward radial direction at the contact location.
 15. Aninterruptible power transmission mechanism for coupling a drive sourceto a driven machine comprising: a first rotor that is driven by thedrive source; a second rotor that rotates synchronously with the drivenmachine; a coupler that connects the first rotor to the second rotor,the coupler includes a coil spring, wherein stress in the coupler variesin accordance with the load torque of the driven machine; and anabutment that contacts the coupler at a contact location when the loadtorque of the driven machine exceeds a predetermined value, wherein theabutment increases the stress in the coupler at the contact location andpromotes breakage of the coupler when the load torque of the drivenmachine exceeds the predetermined value, and the abutment increases theratio of a change in the stress in the coupler to a change of the loadtorque in the driven machine.